DOLOMITIZATION OF MIDDLE MIOCENE BUILDUPS, UM GHEIG AREA, RED-SEA COAST, EGYPT

Middle Miocene reefal carbonates of the Um Mahara Formation at Um Ghei g area near the Red Sea coast of Egypt had a complex history of dolomi tization. Various dolomite petrotypes are recorded including; 1) dolom ite replacing allochems with complete fabric preservation, 2) mimic do lomite replacement for originally Mg-calcite radial fibrous cements, 3 ) fibrous, span, and micrite dolomite cements that fill some secondary pores, and 4) dolomite matrix which possesses non-planar crystal boun daries and polymodal size distribution. The investigated dolomites exh ibit a narrow delta(18)O range (-3.6 to -6.3 parts per thousand PDB) r elative to their delta(13)C values(1.2 to -11.2 parts per thousand PDB ). The petrographic and isotopic characteristics are in accord with mu ltiple phases of dolomitization, related to sealevel variations. The f irst dolomitization phase was an early and pervasive event that took p lace during a stillstand period within the zone of circulating marine pen-fluids ahead of a mixing zone. This resulted in dolomitization of the bioclasts and early Mg-calcite marine cements. Excellent fabric pr eservation of these components is attributed to pre-dolomitization dia genesis. Silica interruption for this early dolomitization phase is in dicated by the sharp contacts observed between the silica minerals and dolomite and the presence of dolomite inclusions within quartz. As se alevel continued to drop, the mixing-zone migrated downward through th e dolomitized reefal body resulting in recrystallization of early dolo mites and precipitation of some dolomite cements of limited extent. Th is late dolomitization phase was followed by meteoric diagenesis as th e ground water continued to migrate basinwards. During this period of diagenesis meteoric calcite cements, that show relatively depleted del ta(18)O and delta(13)C values (-6.6 to -9.3 and -5.3 to -7.9 parts per thousand PDB respectively), were precipitated within some secondary v ugs.